Posted
by
samzenpus
on Thursday June 16, 2011 @11:30AM
from the bigger-and-better dept.

Zothecula writes "Since its completion in 1963, the Arecibo Observatory in Puerto Rico, with a diameter of 305 m (1,000 ft) and a collecting area of 73,000 square meters (790,000 sq ft), has been the largest single-aperture radio telescope ever constructed. But Arecibo is set to lose its title with construction now underway in Guizhou Province in southern China of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Upon its expected completion in 2016, FAST will be able to see more than three times further into space and survey the skies ten times faster than Arecibo."

It's fundamentally not the elephant in the room. An "elephant in the room" is an obvious issue that no-one talks about. The difficulty is in having a single god damn conversation about anything that happens in a country containing 1.3 billion people without it turning into the exact same discussion. Yes China can be a very nasty place in terms of censorship, and yes every single reader of Slashdot knows that. Do we need to hammer it home again?

Why are you trying so desperately to turn any discussion about China into a political one?

Well, why not? Every discussion about the US turns into a long list of complaints about how either a) America sucks and has always sucked, or b) America is giving up its superpower status to the Chinese.

Why are you trying so desperately to turn any discussion about China into a political one?

Well, why not? Every discussion about the US turns into a long list of complaints about how either a) America sucks and has always sucked, or b) America is giving up its superpower status to the Chinese.

America is not "giving up" superpower status to anyone. China is merely rapidly catching up. This is a good thing; a lot of people may point at China and cry out about human rights abuses, etc. but the simple fact of the matter is that human rights abuses are more likely to happen where people are poverty stricken and trying to claw their way out of it. People with a comfortable quality of life are less likely to try to fuck everyone else over to make a buck. And as for any perceived danger from a military

the simple fact of the matter is that human rights abuses are more likely to happen where people are poverty stricken and trying to claw their way out of it.

Wrong. The simple fact is that when humans are too poor to protect their rights, they get abused. Typically more by the people organized enough to get relatively rich from abusing them. Which is why they abuse them.

Don't blame the poor for the human rights abuses when the evidence it's done to the poor by the less poor is everywhere.

I don't disagree with this, but there are many posters on Slashdot that do. I don't think of this as a zero-sum game; China getting richer and more powerful does not have to mean everyone else (or just the USA) getting poorer and weaker. I also think some aspects of China's rise are being overstated, as are some aspects of America's (supposed) decline. But that's not my point: it's clear that a large number of peo

Never having to wonder if you're creating problems or false data when you put everything together has to be worth something. Labor is cheap there (as they can always just criminalize something or more to the point, enforce some of the existing laws to produce a labor pool) and so is space since you don't get paid off for eminent domain even as well as you do here.

For a given surface area, an array is better at looking with high angular resolution. So for mapping a small region, or determining the proper motion of a pulsar, an array is good. But if you want to search the whole sky for pulsars (for example), a single dish of the same area is better because it has a wider field of view with full sensitivity.

So in general, single dish telescopes are better for surveying the sky, arrays are better at observing known sources/objects with high resolution and sensitivity

Single dish should also give a clearer image for the same surface area (less edge, less perimeter you need to seal against terrestrial radio inteference, fewer timing problems since you're not using interferometry). The problem with single dish is steering. Aricebo is fixed for a reason. The telescope at Jodrel Bank observatory, although not the largest steerable dish, is one of the larger steerable telescopes and the infrastructure needed is absolutely staggering.

For those interested in stats, here's the facts and figures [man.ac.uk] for the telescope. If you're not interested in clicking through, the numbers that matter are that the dish is 76.2 meters in diameter and weighs 3,200 metric tonnes. It's also the third-larges

Single dish should also give a clearer image for the same surface area (less edge, less perimeter you need to seal against terrestrial radio inteference, fewer timing problems since you're not using interferometry). The problem with single dish is steering. Aricebo is fixed for a reason. The telescope at Jodrel Bank observatory, although not the largest steerable dish, is one of the larger steerable telescopes and the infrastructure needed is absolutely staggering.

Arecibo is steerable. Its transceiver is mounted on a suspended cradle above the dish, where it is regularly steered by mechanical means for just this purpose.

Different strengths and weaknesses. With an array, you do interferometry, and can get very high angular resolution (the capability to distinguish between very close but separate sources). Look up aperture synthesis [wikipedia.org] for more detail. If the source is bright enough, it's as if you had a virtual antenna with a diameter equal to the baseline (the distance between the individual antennas in the array). You can easily have individual antennas in the array positioned tens of kilometers apart, while a single antenna

Given the 5 to 3 ratio in apertures between the two telescopes, I think that it will be able to "peer" (25/9)^0.5 = 5/3 = 1.67 times "further into space," where "peer" means resolve an object at a given signal to noise ratio. Collected light scales with the square of aperture, but signal to noise ratio only improves with the square root of the number of collected photons. In more useful terms, it should be able to resolve the same thing to the same statistical certainty in 3/5 of the time.

I don't think that photon counts are the dominant noise factor. I believe its thermal receiver noise predominantly, so uncertainties are proportional to the thermal noise divided by the square of the area.

I have no clue about astronomy tech, but in NMR spectroscopy, we usually cool down our high end receiver coils in liquid nitrogen to get rid of some of the thermal noise, and try to get the shortest signal path to the pre-amp possible. I'd be surprised if radioastronomers wouldn't use cryocoil setups in their receivers, too, so maybe the diameter still would be the dominant factor for sensitivity here? Feel free to correct me - it's not my field of experience.

Diameter does matter, it's just that counting statistics aren't the main noise source. Radio photos are so unenergetic that any detection is billions of photons. The receivers are LN2 cooled, and you can get L-band (1.4GHz) system temperatures (the sum of all the noise processes) down to around 25K. The external noise sources add to that. The microwave background is 3K. Atmospheric noise is about 1K. Galactic noise can be significantly higher than that (10s to 100s of K), especially if you're lookin

Thanks for the info. It seems quite similar to NMR then, regarding the signal to noise problem - we are dealing with population differences of 999.999 to 1.000.000 between ground states and excited states here. At least we can exclude most of the external noise, though...

We in the spectral-line radio astronomy world make graphs, each a fuzzy horizontal line with a vertical spike in the middle, representing a spectral line of a molecule. The height of the spike relative to the fuzz on the baseline is that signal-to-noise ratio you're talking about, and it can be on the order of.01% signal/noise. But the telescope integrates that signal (signal+noise minus noise only) over a long time period, resulting in the ability to see a spike at all. As you say, the longer you look at

That's long baseline radio astronony, which would be useless for things like pulsars (there's not a pulsar slow enough to observe the pulses with long baseline) and of questionable value for extrasolar planets close to their sun (the orbit will result in the aggregate signal being worse) but it's great for observing stars, nebulae, gas clouds and 95% of the stuff radio astronomers get excited over.

My point is less to do with interferometry and more to do with the length of baseline used. The longer the baseline, the less fuzz you get but at a cost of losing anything that's time-dependent. Constant signals will clearly be the same value (the average being the same as the maximum and minimum), pulsed signals will show up at a fraction of their strength where the fraction is equal to the fraction of the time the pulse is present AND will not show any pulsation, and so on.

There are all kinds of factors. Imperfections in the dish, for example, will reduce the useful photons collected and increase the noise. Since materials expand and contract with change in temperature, such imperfections will vary with time.

Photons? For a RADIO telescope?? Yes, quite a while since your astro classes.

The SNR you quote, which improves as square root of the collected radiation (radio waves), is voltage SNR. In terms of power, you still get a 9dB SNR improvement for 3 times the collected radio waves, and in these electronics, it is electric power which is the ultimately used. However, the improvement factor should again be raised to power 1/3 to get distance, since the amount of signal power collected by an aperture, after sphe

Photons? For a RADIO telescope?? Yes, quite a while since your astro classes.

Ummm... ALL electromagnetic radiation propagates as photons, not just visible light. What we call light is just a specific range of wavelengths of electromagnetic radiation. In essence it is no different than radio waves, microwaves, X-rays, gamma rays etc.

More radio telescopes are generally a good thing. One of the major tensions in the field now is whether one should have large radio telescopes or lots of comparatively smaller ones that coordinate their work. Both methods have different advantages. Lots of smaller telescopes linked has the major advantage that if some of them go down for some reason one can still do good science. However, the larger ones can have lots of neat technologies. As TFA discusses, this telescope (FAST) will be able to deform its mirrors in real time to focus on sources. That will help a lot for work on faint radio sources.

However, I'm not sure that this is the best use of resources. As discussed in TFA, the Square Kilometre array is being built by a variety of countries working together, and it will do a lot of the same stuff. http://en.wikipedia.org/wiki/Square_Kilometre_Array [wikipedia.org] However, the SKA and FAST will be looking at different regions of the sky, and where they do overlap will be looking at different times. So overall this is helpful. Personally, if I were going to be putting this much resources into interesting Earth-based astronomy, I'd probably want to focus more on increasing our neutrino detectors. We're not investing very much in that, and it is a very new, very interesting field of astronomy/astrophysics. Moreover, neutrino astronomy is pretty much the only thing that can give us warning (albeit only a few hours) if a nasty supernova happens in our vicinity. Right now, that doesn't look likely, but it would be nice to have some warning in case our models are off. Moreover, even without a threat issue, since neutrinos can arrive before the light from a supernova (since the neutrino burst occurs before most of what we would call a supernova, and neutrinos travel at very close to the speed of light), they can help us point our optical and X-ray telescopes in the right regions before we the light reaches us, which is really helpful for advancing our understanding of such events.

Overall though, shouldn't be complaining. It is very difficult to get almost any good funding now for astronomy and cosmology research. In that regard, this is a good thing.

I don't there will be any overlaps. The more dishes the better (think SETI for example). FAST is contributing to the SKA project - scientists and engineers co-operate. The foundation of FAST design was crafted in co-operation with Arebico.

Overlap is actually a good thing, since observations that can only be repeated on the same instrument or same class of instrument cannot be definitely attributed to a cosmological source - it could be explained equally well as a flaw in the design of the instrument. Having two entire classes of radio observatory being able to validate each other will permit testing and validating of the devices.

Moreover, neutrino astronomy is pretty much the only thing that can give us warning (albeit only a few hours) if a nasty supernova happens in our vicinity

There are no candidate supernovae progenitors close enough to us to be dangerous. (I can't remember what the lethal radius is, but it is not terribly large.)

I once tried to find the nearest possible supernova progenitor, and the best I could find was gamma velorum A, at 260 parsecs away, but I didn't spend a long time on this. (I might have been only looki

I really wonder who China's trying to impress with all this. I can only hope no one. There's more than one reason the radio telescope race ended in the 1960's; but it's primarily because no one cares anymore.Maybe they have a use for the telescope, but it doubt it. Instead, it seems they're just trying to waive a penis around because they can instead of doing anything useful....

I dunno. The politics over SKA, where it would be located, etc, show that people do indeed care. Nobody can put a telescope even the size of the Lovell dish into space, never mind the size of this monster. Single dishes have benefits (such as reduced edge effects) that arrays do not, which is extremely important for some of the science needed. Radio telescopes are still the only systems you can build large interferometers from (you can do small optical interferometers, but that's it). RFI is an increasing problem for radio observatories, due to flagrant abuse of the spectrum by many nations, and it's much easier to shield one site than a hundred. Precision-engineering a single dish of this size will require advances in material science that will have spin-off benefits in other fields.

In short, there's lots of reasons for them to do this and no obvious reason for them to copy SKA or SHA.

Worse, Arecibo is deformed, requiring complex hardware to compensate. There's bound to be some loss of quality when trying to compensate. Also, it's an old telescope now. There have even been plans to shut it down, which would likely go through if presented to Congress today. If China had a rival, even if only of equal size, astronomers needing a single dish rather than an interferometer would have no alternative but to buy telescope time from them.

Boy am I tired of the Chinese obsession with size. Everything has to be either the biggest, or the smallest. Think of all the recent headlines along the lines of "China building largest intercontinental rail" or "China building worlds smallest computer chip" or "China building the largest solar power plant" or "China has the smallest currency value" or the largest GDP or the most number of university graduates or the smallest standard of living... why not give other countries a chance?

Nothing's keeping other countries from making advances of their own, except their own stupidity, short-sightedness, and greed. If you want to do something great, then go do it instead of wasting your money on stupid wars and corporate welfare.

What do consider Big Science? Getting a better picture of the universe can be a good thing but it doesn't really contribute much to day to day life but people are still funding, building, operating, and sharing observatories, particle accelerators, solar test farms, and other large scale research level projects for pure academic and scientific study. I think it is amazing that there are still people who fund these types of programs without ever expecting a return on their investments in their life time. In

Believe it or not but not everything needs to devolve into political idealology pissing matches. The rise of the right causing the decline of the US science is a political slogan not a fact. Both the left and the right have their heads up their asses when it comes to anything these days let alone science. Judging by your comments you sound like one of those idealogy blinded neophytes who do nothing but moan about the future and get in the way of people trying to actually live a life instead of just endlessl

I always thought that in radio astronomy you can accomplish more by a lot of small radar dishes linked up than with one single huge one? Wasn't there at some point a plan to send a bunch of sats up that would fly in formation to form a friggin' HUGE (read: several thousand miles) array for radioastronomy?

I would care less about the news of a Chinese large-aperture radio telescope if Arecibo weren't constantly held under the axe of NSF funding cuts. We need at least one operating on the planet to track earth-bound asteroids, among other things, and there's no guarantee the U.S. Congress will keep ours operating.

First of all, it's a Chinese telescope and its name is an English acronym. How fucking stupid is that? And "Five-hundred-meter"? Since when is the first letter of a NUMBER (spelled out in words) significant for the purposes of an acronym? Stupid. And "radio" apparently isn't important enough to be represented in the acronym either. Last (but not least), it isn't even fucking spherical, it's parabolic (like ANY OTHER radio telescope).

Yeah -- and an incorrect one, at that. Not all radio telescopes are parabolic. Arecibo, for instance, is spherical. The original paper [arxiv.org] even notes, "FAST is an Arecibo-type spherical telescope" (p. 3).